Gadenin日本語Get the appOld wood or new wood: the only question that matters
Every pruning decision comes down to which wood the flowers are carried on. Some hydrangeas form next summer's flower buds in late summer of the previous year, on shoots that have already grown. Those are the old-wood group, and anything you cut off in autumn, winter or spring is a flower you will not see. Others form their buds on shoots that grow from scratch in spring, and those can be cut hard every year without losing anything.
Getting this wrong is the single most common reason a healthy, vigorous hydrangea produces no flowers at all. A shrub tidied up in late autumn, or cut back hard in early spring because it looked untidy, will grow beautifully and flower not at all. The plant is not sick and the soil is not the problem.
If you do not know which group yours belongs to, the safest answer is to prune nothing for a full year except dead wood, and watch where the flowers appear. Flowers on shoots that were already there before the season started mean old wood. Flowers at the tips of shoots that grew from the base this year mean new wood.
| Group | Buds formed | When to prune | How hard |
|---|---|---|---|
| Old wood | Late summer, for next year | Straight after flowering | Light: dead heads and thin stems only |
| New wood | Spring, on this year's shoots | Late winter or early spring | Hard: down to a low framework |
| Reblooming | Both old and new wood | Straight after the first flush | Light, and never in spring |
| Climbing kinds | Old wood | Straight after flowering | Only to control size |
If you inherited the shrub and cannot tell, mark a few stems with soft twine in early spring before growth starts. Whichever stems carry flowers tells you the group without any guesswork.
Pruning the old-wood types
These need very little. The job is to remove the spent flower heads and open up the center, not to reduce the size of the shrub. Do it as soon as the flowers fade, and certainly no later than the end of summer, because bud set for next year begins within weeks of flowering finishing.
Cut each spent head back to the first strong pair of buds below it. Those buds are usually plump and obvious an inch or two (3 to 5 cm) down the stem. Cutting lower than that, into the thicker old wood, removes buds that would have flowered.
- Deadhead to the first fat pair of buds Follow the stem down from each faded head until you meet a pair of well-developed buds, and cut just above them. Removing more than that costs flowers, no matter how much better the shrub looks afterwards.
- Take out dead and damaged wood Cut dead stems right out at the base. Dead wood is gray-brown, snaps rather than bends, and shows no green when you scratch the bark. This can be done at any time of year without penalty.
- Thin one third of the oldest stems On a congested mature shrub, remove up to a third of the thickest, grayest stems at ground level. This renews the plant over three years without ever cutting the whole thing back.
- Leave the heads on over winter in cold gardens The dried flower heads shelter the buds immediately below them from frost and wind. In a cold climate, leave them and deadhead in spring once the danger of hard frost has passed.
Pruning the new-wood types
These are the forgiving ones. Because the flowers form on shoots that have not grown yet, you can cut the whole shrub back in late winter or very early spring while it is dormant and still get a full display. In fact you should, because unpruned plants of this group make ever longer, weaker stems that flop under the weight of the heads after rain.
Cut back to a low permanent framework, leaving one to two feet (30 to 60 cm) of woody stem, and cut just above a pair of buds. The harder you cut, the fewer and larger the flower heads; a lighter cut gives more heads at a smaller size. Choose deliberately rather than by accident.
The one caution is stem strength. A shrub cut extremely hard every year produces enormous heads on stems that cannot hold them. If yours flops, cut less hard next season and leave a taller framework, or thin the shoots so the remaining ones thicken.
Do not prune these in autumn. The cut stems die back further over winter and you lose part of the framework, and the shrub wastes energy healing wounds instead of hardening off.
The other reasons a hydrangea does not flower
Pruning is the first suspect but not the only one. A late frost after the buds have broken destroys the season's flowers on old-wood types outright, while the shrub goes on to produce perfect foliage. If your shrub leafed out normally in spring and then simply never flowered, and you did not prune, look back at the date of your last frost.
Too much nitrogen is the second cause. Feed high in nitrogen, or a shrub sitting at the edge of a fed lawn, produces lush growth at the expense of flower buds. Switch to a low-nitrogen feed and keep lawn fertilizer off the root zone.
Too much shade is the third. A hydrangea that flowered well for years and gradually stopped has often been shaded out by a growing tree or hedge. Deep shade gives you leaves and nothing else; aim for at least three to four hours of direct light, ideally in the morning.
Age is the last. A very young plant may take two or three seasons to settle before it flowers well, and a very old, congested one with nothing but thick gray stems needs the renewal thinning described above rather than a feed.
| Symptom | Likely cause | Fix |
|---|---|---|
| Strong growth, no flowers at all | Pruned at the wrong time | Identify the group, prune on the right schedule |
| Leafed out normally, then nothing | Late frost killed the buds | Shelter or row cover over hard spring frosts |
| Very lush, soft, floppy leaves | Too much nitrogen | Low-nitrogen feed, keep lawn feed away |
| Gradual decline over years | Increasing shade | Prune back overhanging growth or move the shrub |
| Few flowers on a thicket of stems | Congested old plant | Thin a third of the oldest stems each year |
What to record before you cut
Write down the date you pruned, which shrub, and how hard you cut, then write down when and how well it flowered the following season. One year of that turns pruning from guesswork into a decision you can defend, and it is the only way to be certain which group an unlabelled shrub belongs to.
Record your last spring frost date as well. If a shrub fails to flower in a year with a late frost and flowers normally in a year without one, you have your answer, and you know to protect the buds next time rather than blaming your pruning.